This invention relates to the field of nondestructive testing. In particular, the invention relates to electronic shearography, which produces interference patterns generated by laterally-displaced images of an object.
Early laser-based systems for nondestructive testing used holography, wherein a beam reflected from the test object is made to interfere with a reference beam. Examples of patents showing holographic systems for performing such testing are U.S. Pat. No. 3,828,126 and U.S. Pat. No. 4,304,458. The major disadvantage of holography is that, by definition, it requires an object beam and a reference beam. Because two beams are needed, the optical arrangement is relatively complex. Moreover, a holographic system is especially sensitive to vibration. For these reasons, holography has been used mainly in the laboratory, and has not been satisfactory as a technique for inspecting manufactured parts in a factory environment.
The technique of shearing interferometry, or "shearography", has been recognized as a preferred alternative to holographic interferometry. In shearography, there is no separate reference beam. Instead, two images of the same object are made to interfere, forming a pattern that can be recorded. A first shearogram is taken while the object is in an unstressed condition, and another shearogram is taken while the object is stressed. Comparison of the two shearograms can reveal information about the strain concentrations (and hence the integrity) of the object.
Various methods have been proposed for using shearography in nondestructive testing. U.S. Pat. No. 4,139,302, the disclosure of which is incorporated by reference herein, is one example. In the latter patent, the shearing is accomplished by a wedge-shaped prism which lies along a portion of a lens. The light rays passing through the prism are displaced relative to those which do not pass through the prism. In this way, the system produces two laterally-sheared interfering images of the object.
While the apparatus shown in the latter patent (and in other comparable references) does produce acceptable results, it is not capable of generating interference patterns in "real time". The interference patterns generated must be recorded on a high-resolution photographic film. To obtain a shearogram, one must wait to develop the film, and must use film-developing chemicals. Thus, a film-based system cannot be used to view the instantaneous status of a test object.
In order to view a "real time" image, one must record the interference pattern with a video camera, or its equivalent. Until recently, it was virtually impossible to use a video camera to record shearograms because the spatial frequency of the interference patterns generated by the then known shearography methods was great, far beyond the resolving power of available video cameras.
The only known workable system for practicing shearography in "real time" is that described in U.S. Pat. No. 4,887,899, the disclosure of which is incorporated by reference herein. The latter patent is a remarkable advance over the prior art discussed above. It discloses a device which forms a shearogram by passing light, reflected from a test object, through a birefringent material and a polarizer. The birefringent material, which can be a calcite crystal, splits a light ray, reflected from a point on the object, into two rays, and the polarizer makes it possible for these rays to interfere with each other. It turns out that the spatial frequency of the shearogram produced with this arrangement is relatively low, because the effective angles between the interfering rays are small. Each shearogram is stored in a computer, or in an equivalent device, and the shearograms taken before and after deformation of the object are compared electronically. The result is a composite interference pattern which graphically shows the condition of the object. By storing the shearogram of the object in its initial, unstressed condition, and by comparing that shearogram, almost instantaneously, by computer, with further shearograms taken under varying levels of stress, a "real time" image of the resultant strains on the object can be observed.
As used herein, the term "electronic shearography" means a shearography process in which the interference patterns are recorded and analyzed electronically, as by a video camera, in contrast to processes which record patterns on photographic film. The above-described patent describes the first known commercially viable method for electronic shearography.
Although the system shown in U.S. Pat. No. 4,887,899 is commercially useful, it still has certain disadvantages. One of these is the inability to control easily the amount of shear (also called the "angle of shear"). The amount of shear is fixed, and is determined mainly by the optical properties of the birefringent material. But it is sometimes necessary to vary the amount of shear; a smaller angle of shear (i.e. in a system having reduced sensitivity) is preferable in a high-vibration environment, and conversely. To change the amount of shear, in the above-described system, one must replace the optical element. Also, the above-described system is subject to losses caused by attenuation of the light beams as they pass through the birefringent material and polarizer.
The present invention provides a new means of practicing electronic shearography. The invention solves some of the problems mentioned above. In particular, the invention makes it easier to adjust the amount of shearing, and thereby to adjust the sensitivity of the system, without replacing the optical element. The invention also tends to reduce the power requirements for the illuminating laser, because it minimizes the attenuation of the light reflected from the test object. The invention also allows the reflected beam to be phase-stepped, thereby providing more detailed information about the imaged strains.